Window balance systems using resin jamb liners with spring covers have generally limited the spring covers to the upper half of each jamb liner and have let sash platforms move up and down with the sashes in the bottom halves of the jamb liners where no spring covers interfere with platform movement. The Suess U.S. Patent 4,570,382 has suggested full length spring covers extending from top to bottom of the jamb liners with open vertical slots through which sash platforms can extend into adjustable friction shoes that ride up and down inside the spring covers. The spring covers require extra internal walls providing tracks for the friction shoes to work against.
Instead of an open slot and extra internal walls in a spring cover, as suggested by Suess, I have discovered a way that a platform can work with a spring cover that is simply slit so that the platform and the spring cover can achieve the friction necessary to prevent hop or drop of the balanced sash. My friction system not only uses a less expensive construction, but also provides a frictional resistance that is variable as a function of the spring force pulling up on the platform and the weight of a sash resting on the platform.